Recently, new image forming means such as color electrophotography, ink-jet ink and color filter are proposed and organic dyes becomes to be used in various fields.
As properties required to the dyes to be used for color image formation, suitable light absorbing property with sharp absorption, low sub-absorption and high light absorption coefficient from the viewpoint of color reproduction, high stability against heat and light, and solubility in a solvent according to the use are cited. It is present situation that sufficient dye is not found out yet though various developments have been carried out for satisfying such the requirements.
As disclosed in various documents, cf. Patent Documents 1 and 2 for example, the electrophotographic method is a method in which an electrostatic latent image is formed by various methods on a photoreceptor containing a photoconductive substance and the latent image is developed by a toner to form a powder image and the powder image is transferred onto paper according to necessity and then fixed by heating, pressing or solvent vapor.
Recently, a color copying method is made practicable, in which an electrostatic latent image of the original is formed by exposing by spectrally separated light and developed by a colored toner to form a colored image and the images different in the color from each other are overlapped to obtain a full color image. As the toner to be used in such the method, toners each colored in yellow, magenta or cyan are produced which are each constituted by a binder and a pigment of and/or dye of each of the colors dispersed in the binder.
By the electrophotographic method, an image is generally formed by the following processes.
Firstly, an electrostatic latent image is formed on a photoreceptor by irradiating light information corresponding to the image information onto the photoreceptor constituted by a photoconductive substance by various methods. After that, the electrostatic latent image formed on the photoreceptor is developed into a toner image by an electrically charged toner, and the toner image is transferred onto an image recording medium such as ordinary paper or an intermediate transferring member and fixed onto the paper using a thermal fixing device.
In the above color image forming method using the electrophotographic system, the electrostatic images formed on the photoreceptor is corresponding to information of images each separated in the colors of yellow, magenta, cyan and black, respectively, and developed by the toner having the color the same as that of the image information. The color image is formed by repeating for four times such the developing for each of the colors.
Hitherto, organic pigments and dyes are used as the colorant to be used for the electrophotographic toner but they each has various drawbacks. For instance, the organic pigments are generally superior to the dye in the heat resistivity and light fastness but the pigments have high covering power and the transparency of them is lowered since they are in a dispersed state. Moreover, the transparency is spoiled and chromaticness is lowered so that the color reproduction of the image is degraded since the dispersibility of the pigment is generally insufficient.
High transparency of the fixed toner is necessary for making possible to observe the color of the toner of the lowest layer without covering by the upper layer and the dispersibility and coloring ability of the colorant is required for holding the color reproducing ability of the original image.
As the method for dissolving the drawbacks of the pigment, a means by which a dispersed pigment particle diameter of submicron order of primary particles without coagulated secondary particles is attained by using a flashing method for improving the transparency and a means by which the electric charging ability, fixing ability and image uniformity are improved by covering the pigment particle by a binder resin and a shell resin are proposed, cf. Patent Document 3. However, sufficient transparency is difficulty obtained by the use of pigment toner even when the image is output by the above proposed toner.
Entire colors can be reproduced in principle by subtractive color mixing of three colors of yellow, magenta and cyan by the color image forming apparatus. In practice, however, many problems are leaved for correctly reproduce the color of the original image since reproducible range and the chromaticness of the color are lowered depending on the color mixing ability when the toners different in the color are overlapped.
On the other hand, a toner using a dye, cf. Patent Document 4, and that using a mixture of dye and pigment are disclosed. However, the toner using the dye is superior in the transparency and the chromaticness because the dye is in the dissolved state in the binder resin but has drawbacks such as that the light fastness and thermal resistivity are considerably inferior to the pigment.
Relating to the thermal resistivity, problems are caused that contamination in the apparatus by the sublimation of the dye tends to be caused on the occasion of fixing by a heating roller and the dye is dissolved in silicone oil to be used on the occasion of the fixing and finally adheres onto the heating roller so as to cause offset phenomenon additionally to the density lowering by the decomposition of the dye. For dissolving such the drawbacks of the dye, a means for making compatibility of the light fastness, sublimation and the color reproducibility, a means by using a specific anthraquinone dye or a chelate dye is proposed, cf. Patent Document 5. In the case of the toner using the dye, however, sufficient thermal resistivity (sublimation property), light fastness and color reproducibility can be difficultly obtained even when the image is output by using the toner proposed in the above, and the development of a toner satisfying the above conditions is demanded.
It can be considered that the properties are improved by adding an additive, cf. Patent Document 6. However, the stability of the additive itself is required in such the case. In all the cases, the stability of the dye and the additive is important required item but sufficient dissolving means is not obtained yet.    Patent Document 1: U.S. Pat. No. 2,297,691    Patent Document 2: JP B S42-23910    Patent Document 3: JP A H11-160914    Patent Document 4: JP A H05-11504    Patent Document 5: U.S. Pat. No. 3,567,403    Patent Document 6: JP A 2007-34254